Retire QTest::pixmapsAreEqual().

Retire QTest::pixmapsAreEqual().

QTest::pixmapsAreEqual() was left in the Qt4 API for compatibility with
some old tests written for Qt3.  QCOMPARE() is the preferred way to
compare QPixmaps and provides superior diagnostic output when a
comparison fails.

This commit removes QTest::pixmapsAreEqual() from the testlib API and
replaces the last few remaining calls with QCOMPARE.

Change-Id: I051c0e7d3bda072855fcd262d82e8e540619233b
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>